Low amplitude signals are frequently obscured by the noise generated by a sensor of the signal. Thus, small changes in the actual signal are masked. One way around this problem is to extend the dynamic range of the signal by amplification.
One such signal having a small change in amplitude relative to the underlying background signal is generated by photoplethysmography (PPG) measurements. In these measurements, light is used to measure the changes in vascular blood volume generally in a peripheral part of the body. The volume of blood in that body part, such as a finger, changes during each heartbeat.
In a typical embodiment of a photoplethysmographic system 10 (FIG. 1), light from a light source 14, such as a light emitting diode 14, is either transmitted through or reflected from the internal vasculature (only through shown for clarity) and detected by a photodetector 22. Generally the light source 14 actually comprises a red LED and an InfraRed (IR) LED (not shown). The signal from the photodetector 22 is amplified by an amplifier 24, demodulated by demodulator 26 so that the signals from the red and IR LEDs can be separated, low-pass filtered 28, and converted to digital form by an analog-to-digital converter 30. The digitized signal is processed by a microprocessor 32 and the PPG measurements displayed by an output device 34. The microprocessor also determines when the LED lights 14 should be turned on and controls the illumination using an LED driver circuit 36. The electrical signal from the photodetector varies over the heart beat cycle and is analyzed by the processor using signal processing techniques.
One way known to the prior art to increase the dynamic range of the signal from the photodetector is shown in FIG. 2. This embodiment involves using an analog error amplifier 40 in a feedback loop with a main amplifier 44 and photodiode 38. This error amplifier 40 controls a transistor 42 which generates a current that cancels the static current of the photodiode 38. The error amplifier 40 tries to maintain the output of the main amplifier 44 at a specific voltage that is typically at the midsupply voltage of the system. The pass-band of the circuit is dependent upon the amount of current sourced by the transistor. Because PPG signals are typically low frequency, it is difficult to generate a pole close enough to DC in an integrated circuit to make the pass band of the circuit begin at 1 Hz. Thus, these systems frequently infringe on the bandwidth of interest and are inherently lossy.
What is needed is a system that permits expanded dynamic range while having components that permit it to be constructed in a small package such as an integrated circuit for portable use.